An active heave compensator usually comprises a linear actuator, typically a hydraulic cylinder construction with three chambers. This construction has typically axial movement of about 8 meter. The actuator could also be of another form of linear actuator, such as electric powered. This active compensator actuator is generally related to a passive heave compensation machine, called drill string compensator or passive compensator. The passive compensator exploit the gas compressibility of air or nitrogen in large pressurized containers (APV's) which are hydraulically connected to one or more hydraulic cylinders via piston accumulator(s). The piston accumulator(s) is acting as a phase separator between the pressurized gas volume and the oil-filled hydraulic cylinders. The hydraulically cylinders balance thereof the load in this passive compensator and offsets most of the vessel heave motions using spring force from the pressurized gas volume. A typical drill string compensator manipulates the load in that the hydraulic cylinders in the passive compensator are mechanically connected to a movable crown block in the draw work system of the derrick. The actuator of the active compensator is also connected to and manipulates actively the movable crown block with a smaller force that helps to reduce the load fluctuation and deviation position from the passive compensator in that it outweights the friction in the passive compensator. A control system regulates the force—and position load based on real time measurements from an acceleration sensor (MRU) on the vessel. In this way the performance of the total heave compensator system are improving in that all heave induced influences are eliminated. This is important when delicate operations are carried out on the seabed or in the well.
In drilling rigs which are equipped with top-mounted active heave compensation actuator and passive heave compensator, the actuator of the active compensation is usually connected permanently to the movable part of the passive compensation machine.
The total amount of time the drilling rig is operational with active heave compensation is often limited compared with the time it is operational with passive heave compensation alone. The actuator of the active compensator is hence driven out and in by the spring force in the passive compensator in a greater portion of the time.
To minimize the abrasion and achieve optimal operation, it is advantageous to have an arrangement where the actuator of the active compensation can be connected and disconnected when it is needed. Such an arrangement will increase the lifetime of the active compensator actuator and increase the capacity of the passive compensator.